1. Technical Field
The present invention relates generally to digital signal transmissions and in particular to equalizers. Still more particularly, the present invention relates to an improved decision feedback equalizer.
2. Description of the Related Art
The advent of the information age has increased the demand for the storage of digital data, along with the demands for processing and transmission of such data. The density of information stored in a single system has been increased to accommodate the growing demand. For example, the capacity of magnetic disks storage units has grown fueled by improvements in the design of heads and disks, improvements in magnetic media, decreases in head gap length and flying height, and improvements in servo accuracy for increased track density.
The growing demand for digital storage capacity has also prompted an interest in the use of digital signal processing methods as a means of continuing increases in density. The general similarity of read and write processes in disk and tape drive units to data detection in transmission and communication systems has focussed interest on the application of equalization and coding methods to channels for both tape and disk drive systems. In particular, read channels have received special attention because the information has to be processed faster. Equalizers are used in both tape and disk drive read channels. In particular, decision feedback equalizers have been used to recover digital signals in read channels. Two types of noise dominate magnetic recording and read channels. One is regular noise caused, by thermal noise in electronics and random variations in the magnetic media. The other type of noise is intersymbol interference (ISI). Regular noise is found everywhere and effects all channels. ISI becomes worse with increasing density. Pulses generated by transitions in the magnetic media tend to overlap as transitions become closer with higher recording densities. Many of these decision feedback equalizers incorporate finite impulse response (FIR) filters to reduce errors caused by interference between successive pulses of data. This interference is also known as intersymbol interference (ISI). Additionally, errors may occur when the peas of positive and negative pulses do not have the same magnitude.
Presently available equalizers used in read channels contain coefficients that are determined to reduce errors in data transmission (e.g., ISI). The presently available adaptive equalizers adapt or alter coefficients based on an error signal derived from errors in the output of the read channel. These equalizer coefficients, however, are not optimal for all possible types of data patterns. Therefore, it would be advantageous to have an improved apparatus for equalizing signals to reduce errors in transmission of data.
The apparatus of the present invention includes a first filter means for adjusting an input signal based on past data output from the apparatus. In addition, a summing means is used to sum signals from the first filter means and from a second filter means to produce a summed signal. The apparatus includes a symbol detection means for generating an output signal from the summed signal. The second filter means provides adjustments in the output signal based on the peaks and polarity of past signals generated by the symbol detection means. A control means is included for controlling the filtering properties of both the first and second filter means, wherein the control means controls the filtering properties based on the past output signals from the symbol detection means.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.